It is known that an optical fibre is constituted by a core with a refractive index N surrounded by a cladding with an index n lower than N and that it guides, in the core, light which can, for example, be modulated for telecommunications purposes. It would then be advantageous to have a coupler by which, without cutting the optical fibre, light could be inserted into the fibre or a fraction of the light which the fibre is guiding could be extracted, so that the data which the fibre is transporting would be available near the coupling point. Despite this evident need, no really effective device of this type has been proposed. Such a device must apply in particular to multimode fibres. These fibres are those in whose core light propagates in several distinct modes. They are the only fibres which are envisaged for practical industrial use (because of their relatively large diameter 0.15 mm for example with the cladding). The diameter d of the core satisfies the equation: d.f. .sqroot.(N.sup.2 -n.sup.2)&gt;0.7656c where f is the frequency of the light used and c is the speed of light in a vacuum.
A known coupler is described in U.S. Pat. No. 3,931,518 (Miller). In accordance with this patent, with a view to extracting light, the high propagation constant modes which propagate in the core are initially coupled with the low propagation constant modes which propagate only in the cladding.
The propagation constant is changed by forming alternate bends in the fibre by clamping against a grating of parallel grooves cut in a transparent block. Indeed, it is known (D. Marcuse: Coupled Mode Theory for Round Optical fibres, Bell. Syst. Tech. J. 52, p 817-1973) that, when the position of the axis of the fibre, its curvature, its refractive index, or the diameter of the core fluctuate along the propagation axis there is an exchange of energy between the different modes of different values of propagation constant K.
More precisely, if the defect thus produced is sinusoidal with a space frequency P, in radians per unit length, energy is exchanged between two modes of propagation of constants K1 and K2 so such that K1-K2=P; energy can be exchanged in both directions (W. J. Stewart: Mode conversion due to periodic distortions of the fibre axis", Optical Fibre Communications Conference, Sept. 16-18, 1975).
The upper limit KM and the lower limit Km of the propagation constant K of the modes which are liable to subsist in the core are given by the equations: EQU cKM=6.283 fN
and EQU cKm=6.283 fn
where c is the speed of light in a vacuum and f is the frequency of the light used. The coupling of a significant fraction of the transported energy results firstly from a sufficient amplitude of fibre deformation and secondly from the fact that a succession of alterante bends induces a sufficient number of mode transpositions in light which propagates in the core to obtain a final propagation constant in the core below the lower propagation limit. The light whose propagation constant has thus been reduced propagates thereafter in the cladding. It can easily be extrated by an "index adapter" constituted by a transparent medium which has a refractive index which is at least substantially equal to that of the cladding and which is in optical contact with the outer surface of the cladding. The index of the adapter must not be lower than 0.8 times that of the cladding. This adapter is constituted in the Miller patent cited above by the coupling disk 18. Light enters the adapter and is transmitted to an output device such as a photosensitive diode.
Another device for extracting light from a fibre is described in a paper entitled "Directional coupler for single multimode optical fibre" given by C. and W. J. Stewart, at the second European conference on optical fibre transmission held in Paris from 27th to 30th September, 1976 and published by the "Comite du colloque international sur les transmissions par fibres optiques", 11, rue Hamelin, 75783 PARIS Cedex 16 (Cables and connections, Part 2, p 267-268).
In this device, the index adapter is constituted by a thin plate made of a transparent material one of whose edges has an undulation which constitutes a grating and is applied against the fibre so as to deform the fibre periodically. The optical contact between this edge and the cladding of the fibre allows light to pass into this thin plate in which it flows towards a curved edge which reflects it and focusses it onto a detector placed in contact with another edge.
The disadvantage of these various light extraction devices is that they divert only a small fraction of the light which flows in the fibre to the output unit (detector). This fraction is that which leaves in a half-plane which starts from the axis of the fibre, or more exactly in the small dihedral angle formed by two half-planes which start at the axis of the fibre and are very close to each other. In the case of the device described in the Miller patent, the light exit plane is a plane which passes through the axis of the fibre and is perpendicular to the surface of the coupling disk 18. In the case of the device described in the article by Stewart, the light exit plane is that of the thin plate. It is also an advantage to produce a coupling device which inserts light in the fibre with high efficiency. This problem arises in particular when it is required to insert into the fibre as great a fraction as possible of the light produced by a light emitting diode. It is known, for this purpose, to cut the fibre and to concentrate the light which comes from the diode by means of a lens on the cut end of the fibre. The efficiency of such a light insertion device is very poor since the diode emits light from an emitting surface which is not small and over a solid angle which is large. If a lens is used which receives all the light emitted by the diode and which concentrates it entirely on the surface of the cut fibre core, the greater part of this light is at too great an angle with the axis of the fibre for this light to be able to propagate in the fibre. This disadvantage cannot be overcome by any conventional optical system because of a law which is well-known in optics, sometimes referred to as the "theorem of conservation of the geometrical dimensions of a beam". This law states that no optical system can reduce the product of the solid angle of divergence of a beam and the cross-sectional area of the beam without loss of light. This law is expressed in particular in equation 54 on P. 120 in the book "Principle of Optics" by M. Born and E. Wolf (3rd edition, published by Pergamen Press). Taking into account the characteristics of known light emitting diodes, it is observed that the insertion of their light into a fibre is always poor. The efficiency of insertion of light into the fibre can be increased considereably if light of a laser is used which has very little divergence. But other disadvantages, such as the price of the laser, its bulk, etc., then become apparent.
These known light injection devices also have the disadvantage of having to cut the fibre.
The present invention aims to produce an optical coupler for an optical fibre by which high coupling efficiency can be obtained, in particular for coupling with a light emitting diode which emits light over a large solid angle without any necessity for cutting the optical fibre.